PROJECT SUMMARY Two homologous phosphatases, Sts-1 and Sts-2, have been established as negative regulators of signaling pathways within cells of the mammalian immune system. The Sts proteins are characterized by a distinct tripartite structure consisting of two protein-interaction domains and a C-terminal 2H-phosphatase domain (StsHP). They are structurally and enzymatically very distinct from other intra-cellular phosphatases that regulate immune signaling pathway. Recently, Sts-/- mice were evaluated for susceptibility to infection by Francisella tularensis, a gram-negative bacterial pathogen that is the causative agent of tularemia. F. tularensis is of such high virulence that it is classified as a Tier 1 Select Agent by the United States government, along with other potential agents of bioterrorism. Following supra-lethal infectious doses of F. tularensis, Sts-/- mice displayed profound resistance and prolonged survival. The Sts phenotype was associated with an altered inflammatory response in peripheral tissues and enhanced pathogen clearance. Studying the role of the Sts proteins in the anti-bacterial immune response is expected to identify new opportunities to optimize host immune responses toward a deadly human pathogen. Our long-term goal is to develop strategies that promote effective anti-microbial immune responses without accompanying deleterious immuno-pathology. We hypothesize that the Sts phosphatases regulate inflammatory signaling pathways that control mononuclear phagocyte responses to severe bacterial infections, such that their functional inactivation results in a distinct inflammatory environment that favors enhanced clearance. The objective of this study is to establish the role of the Sts proteins in regulating the cellular and molecular anti-bacterial immune response. We plan to test our hypothesis and accomplish our overall objective by completing the following Specific Aim: Specific Aim 1: Define the role of the Sts proteins in regulating monocyte inflammatory responses to systemic Francisella infection. Specific Aim 2: Define the role of the Sts proteins in regulating the microbicidal responses of marrow-derived monocytes. This proposal is innovative because it will reveal a novel regulatory mechanism controlling the immune response to a deadly bacterial pathogen. The proposed studies are significant because their completion will promote the development of novel immune-enhancing therapeutic strategies to improve clinical outcomes for the treatment of lethal microbial infections. Thus, long-term success of this work has the potential to significantly reduce the morbidity and mortality attributed to many bacterial pathogens.